As the title says

The issue has occurred since 24w05a, and can occur even if the world has been played in a newer version in chunks that haven't been loaded since before 1.9.

Steps to reproduce:

1. Generate a world in 1.8.9 or earlier

2. Open the world in 24w05a or later

Observed result:

Many chunks get regenerated. Which chunks get regenerated remains consistent when loading a backup, but I can't see any pattern in which ones do.

Provided screenshots show a world in 1.8.9, with a layer of glass added for visualization, and the same world after loading in 24w06a, showing holes in the glass where chunks were regenerated.